1. Filed of the Invention
The present invention relates to an image forming apparatus, particularly to an image forming apparatus that develops an electrostatic latent image with toner.
2. Description of Related Art
In a conventional image forming apparatus, a developing roller supports a developer composed of magnetic carriers and non-magnetic toner, and supplies the non-magnetic toner to a photosensitive drum with an electrostatic latent image formed thereon so as to develop the electrostatic latent image. In the image forming apparatus, a developing bias is applied to the developing roller so as to form an electric field for movement of the toner from the developing roller to the photosensitive drum.
With respect to the developing bias application, there are two methods, namely, a DC application method and an AC application method. In the DC application method, a DC voltage is applied as the developing bias, and in the AC application method, a superimposed voltage of a DC voltage and an AC voltage is applied as the developing bias. The AC application method permits more faithful development of the electrostatic latent image on the photosensitive drum than the DC application method. Accordingly, an even and smooth toner image (a toner image with favorable graininess) can be formed by the AC application method.
FIG. 5 is a graph showing the AC application method, the relationship between the frequency of the AC developing bias voltage and the graininess. The x-axis shows the frequency of the AC developing bias voltage, and the y-axis shows the graininess. The graininess represents unevenness of a toner image. High graininess means that the toner image has unevenness. FIG. 5 shows cases wherein halftone patches with screen ruling of 210 lpi were formed. The screen ruling represents the halftone dot fineness (how many dots are in a square inch) and is measured in lines per inch (lpi).
As is apparent from FIG. 5, as the frequency of the AC developing bias voltage becomes higher, the graininess becomes lower, and the picture quality of the toner image becomes higher. Therefore, in terms of graininess, it is preferred that the frequency of the AC developing bias voltage is high. More specifically, by setting the frequency of the AC developing bias voltage to 5 kHz or higher, unevenness in a visible degree can be prevented.
FIG. 6 is a graph showing the relationship between the frequency of the AC developing bias voltage and the edge density. The x-axis shows the frequency of the AC developing bias voltage, and the y-axis shows the edge density. The edge density means the toner density at an edge of a toner image in the main-scanning direction. A high edge density means that the toner image has a large difference in toner density between the edge portions in the main-scanning direction and the center portion in the main-scanning direction. (The difference in toner density between the edge portions in the main-scanning direction and the center portion in the main-scanning direction will be hereinafter referred to as a density difference.) Accordingly, when the edge density is high, the picture quality of the toner image is low. FIG. 6 shows cases wherein halftone patches with screen ruling of 210 lpi were formed.
As is apparent from FIG. 6, as the frequency of the AC developing bias voltage becomes lower, the edge density becomes lower, that is, the density difference becomes smaller, and accordingly, the picture quality of the toner image becomes higher. Therefore, in view of a density difference, it is preferred that the frequency of the AC developing bias voltage is low. More specifically, by setting the frequency of the AC developing bias voltage to 5 kHz or lower, a visible density difference can be prevented.
In terms of both the graininess and the density difference, as described above, the frequency of the AC developing bias voltage is preferably 5 kHz.
In conventional image forming apparatuses, generally, the screen ruling is set, depending on the kind of an image to be printed, such as a character image, a photo image or the like. Specifically, when a character image is to be printed, it is necessary to lay weight on reproduction of sharp edges of characters, and therefore, the screen ruling is set high. On the other hand, when a photo image is to be printed, it is necessary to lay weight on smooth gradation expression, and therefore, the screen ruling is set low. In conventional image forming apparatuses, when the screen ruling is changed, it is difficult to keep both the graininess and the density difference in favorable degrees.
FIG. 7 is a graph showing the relationship between the frequency of the AC developing bias voltage and the graininess. The x-axis shows the frequency of the AC developing bias voltage, and the y-axis shows the graininess. FIG. 8 is a graph showing the relationship between the frequency of the AC developing bias voltage and the edge density. The x-axis shows the frequency of the AC developing bias voltage, and the y-axis shows the edge density. FIGS. 7 and 8 show cases where halftone patches with screen ruling of 190 lpi were formed and cases where halftone patches with screen ruling of 210 lpi were formed.
As is apparent from FIG. 7, when a toner image is formed under the conditions that the screen ruling is 190 lpi and that the frequency of the AC developing bias voltage is 5 kHz, the graininess is high, and unevenness is observed in the toner image. In order to avoid this trouble, when the screen ruling 190 lpi, the frequency of the AC developing bias voltage shall be set to 8 kHz or higher.
As is apparent from FIG. 8, when a toner image is formed under the conditions that the screen ruling is 210 lpi and that the frequency of the AC developing bias voltage is 8 kHz, a density difference is observed in the toner image.
As described above, in conventional image forming apparatuses, it is difficult to control both the graininess and the density difference.
As a conventional image forming apparatus, for example, an image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 7-325468 is known. The image forming apparatus changes the frequency of the developing AC bias voltage in accordance with the resolution and the screen ruling so as to perform stable image formation less affected by environmental changes and the usage history. Specifically, in the image forming apparatus, when the screen ruling is high, the frequency of the AC developing bias voltage is set high, and when the screen ruling is low, the frequency of the AC developing bias voltage is low. Therefore, the image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 7-325468 cannot control both the graininess and the density difference.